Effects of Black Soldier Fly (Hermetia illucens) by-product application on fungus gnat performance

Abstract

As production of fishmeal, being a feed source for the growing aquaculture sector, is declining due to the consequences of overfishing, rearing of the Black Soldier Fly (BSF; Hermetia illucens) for producing substitutes is on the rise. Thus, by-products of BSF rearing (frass, pupal cases, imagines) will make up considerable amounts as the sector grows, and the use as organic fertilizer is a promising option for the valuation of these side streams. Chitin is an essential component of fungi and insects and is contained in the mentioned BSF by-products as well. Soil amendment with chitin has been observed to increase chitinolytic activity of the soil microbial community, which might affect soil-living stages of agricultural pests.
In this study, fungus gnats, a horticultural pest feeding on soil-living fungi and plant roots, were exposed to potting soil amended with different BSF by-products (native/autoclaved frass; frass mixed with pupal cases and imagines), and their performance (substrate choice, duration of development, and mortality) was assessed in a series of experiments. To clarify the observed effects, the substrate mixtures were incubated to monitor the time course of chitin content and chitinase activity.
Soil amendment with BSF by-products reduced the generation time of fungus gnats, which might be a consequence of increased growth of fungi, being their main feed source. Despite their shorter generation time, population size was greatly reduced com-pared to pure potting soil. As the autoclaved treatment performed worse, effects may not only be directly related to chitin but may also depend on microbial activity.
With BSF by-products being effective against fungus gnats, they could be included into horticultural potting substrates. Effects on other soil-living organisms and its further po-tential in integrated pest management might need to be assessed as well.